1. Field of the Invention
The present invention relates to a shadow mask for a color cathode ray tube (CRT), and more particularly to a shadow mask for a color CRT that improves the color purity at corners of a screen by varying a vertical pitch of the shadow mask.
2. Description of the Conventional Art
FIG. 1 is a diagram illustrating a color CRT. As shown therein, the color CRT includes a panel 2 to an inner surface of which a fluorescent screen 1 of red (R), green (G) and blue (B) is applied, a funnel 3 attached to a rear portion of the panel 2, an electron gun 5 mounted into a neck 4 of the funnel 3 and emitting electron beams 6, a deflection yoke 7 deflecting the electron beams 6 emitted from the electron gun 5 to an entire area of the screen, a shadow mask 8 sorting colors of the electron beams, and a frame 9 supporting the shadow mask 8 to have a regular interval with the inner surface of the panel 2.
FIG. 2 is a diagram detailedly illustrating the shadow mask 8. as shown therein, the shadow mask 8 has a plurality of apertures 10 to transmit the electron beams 6. The apertures in the shadow mask 8 are arranged through the entire area of the shadow mask from a center to peripheral portions of the shadow mask 8 at a regular vertical pitch Pv or a variable vertical pitch Pv which is determined in a predetermined function.
FIGS. 3A and 3B respectively illustrate an effective area of a shadow mask, wherein FIG. 3A illustrates a shadow mask having apertures arranged at a uniform vertical pitch and FIG. 3B illustrates a shadow mask having apertures arranged at a variable vertical pitch. Here, in each of FIGS. 3A and 3B, O(0,0) is a center coordinate of the shadow mask, E(Xe, Ye) is an effective area and point of the shadow mask, Pv0 is an initial value of a vertical pitch at the center point O(0,0) of the shadow mask, and Pvc indicates the vertical pitch at a corner of the shadow mask.
In the color CRT having the shadow mask in the conventional art, when power is supplied, the electron beams 6 emitted from the electron gun 5 are deflected to the whole area of the screen. While being transmitted through the dots 10 of the shadow mask 8, the deflected beams 6 are sorted and strike the fluorescent screen 1, so that an image is reproduced. Here, since the electron gun 5 is an in-line type and the shadow mask 8 and the panel 2. are gently curved, when striking the fluorescent screen 1 provided at the inner surface of the panel 2, the electron beams 6 emitted from the electron gun 5 strike pixels R, G and B being slanted at an angle of predetermined degrees, as shown in FIG. 4, such a phase being called an electron beam tilt.
In the electron beam tilt, a tilt angle enlarges as it goes towards corners of the screen. Here, the greater the tilt angle becomes, the closer becomes a distance Prb between an electron beam striking a red fluorescent body and an electron beam striking a blue fluorescent body so that although the electron beam slightly erroneously lands, it easily invades another fluorescent body, which results in the deterioration of color purity. That is, the distance Prb between the electron beam striking the red fluorescent body and the electron beam striking the blue fluorescent body is so close compared with a distance between centers of the electron beams that strike green fluorescent bodies, that the electron beam easily invades the other fluorescent bodies, thus lowering in the color purity.
In recent years, an electric field of the CRT has been made small. Therefore, the electron beam tilt angle at each corner of the screen becomes larger. Accordingly, as the tilt angle of the electron beam enlarges, the color purity of the corners of the screen is seriously worsened.
Accordingly, the present invention is directed to a shadow mask of a color CRT which obviates the problems and disadvantages in the conventional art.
An object of the present invention is to provide a shadow mask of a color CRT that prevents deterioration of color purity at corners of a screen.
To achieve these and other advantages and in accordance with the purpose of the present invention, as embodied and broadly described herein, in a shadow mask for a color cathode ray tube (CRT) with plurality of apertures through which electron beams from an electron gun can pass which has a regular interval with an inner surface of a panel by a frame and sorts colors by the electron beams, there is provided an improved shadow mask for the color CRT, wherein a vertical pitch of the apertures of the shadow mask is defined by a plurality of increase functions from a center of the shadow mask to an edge of an effective area thereof, so that the vertical pitch increases towards the effective area end point.